1. Field of the Invention
The invention relates to an optical element molding die for molding an optical element having a concave-convex structure and to the optical element.
2. Description of the Related Art
Optical films with different refractive indexes, such as antireflective films, have been provided individually or in a plurality thereof to a thickness of from several tens to several hundreds of nanometers on the surface of optical elements, thereby making it possible to obtain the desired optical properties. Vacuum film formation methods such as vacuum deposition and sputtering or wet film formation methods such as dip coating and spin coating are used to form these optical films on the surface of optical elements.
Optical elements called SWS (Sub-Wavelength Structure) that have a microperiodic structure have been actively studied in recent years as optical elements having the desired optical properties. Antireflection function is known as a specific feature of optical elements having a microperiodic structure. The antireflective function is realized by providing a periodic structure with a period less than an incident wavelength on a substrate. In recent years, the advancements in microprocessing technology made it possible to form extremely fine and complex patterns.
For example, such patterns are fabricated in a semiconductor process centered on photoluminescence. In this method, a photoresist is coated on a substrate that is used to form a concave-convex structure, exposure and development are conducted via the photomask, a resist mask pattern is obtained, and the mask pattern is transferred by etching onto the substrate for forming the concave-convex structure. Further, a large number of researches have also been conducted to attempt the realization of a concave-convex structure on the base of a naturally formed regular structure, that is, a structure that is formed in a self-regulated manner. For example, a method in which an optical element having a concave-convex structure is manufactured at a low cost by arranging microparticles has been suggested.
An anodic oxidation method is also known as a method by which a concave-convex structure can be formed over a large area at a low cost, and the aspect ratio can be randomly controlled. With this method, microholes are formed by using a metal such as aluminum as an anode in an oxidizing electrolytic solution, passing an electric current therethrough, and causing oxidation. A procedure using this method to arrange regularly the holes side by side has been developed. For example, a method has been developed for producing an optical element molding die by forming an Al film by sputtering on a die having a predetermined shape and then forming holes by anodic oxidation and obtaining a concave-convex structure, as described in U.S. Pat. No. 7,268,948. This process is effective for providing a concave-convex structure, while maintaining a highly accurate surface shape of a lens or the like.
A method for manufacturing an optical element molding die by using the conventional anodic oxidation method is a manufacturing method that effectively makes it possible to form a concave-convex structure over a large surface at a low cost and control randomly the aspect ratio. In particular, as described in U.S. Pat. No. 7,268,948, a method is effective in which an optical element molding die is fabricated by forming an Al film by sputtering on a die a predetermined shape and then forming a concave-convex structure by anodic oxidation. For the dies that are used for high-precision molding of lenses or the like, a Ni processed layer has been used most often due to good processability and stability in molding, and a concave-convex structure produced by anodic oxidation can be formed on the surface, while maintaining the surface accuracy of the die, by forming an Al film on the processed Ni surface and conducting anodic oxidation. However, dust that is generated during processing or dust from the atmosphere adheres to the processed Ni layer. The amount of this dust can be reduced by cleaning after processing, but the dust is difficult to remove completely. As a result, the Al film is formed on the Ni layer on which the adhered dust is present. Ni has a negative standard electrode potential in the oxidation reaction, and when anodic oxidation is conducted, nickel is subjected to anodic electrolysis in the electrolytic solution. The dust that has adhered to the Ni layer is also electrolyzed.
Where the dust is dissolved by the electrolysis or oxygen is generated, the oxidation state of Al differs from the usual oxidation state. As a result, the desired microshape cannot be obtained. Another defect is that external appearance changes locally due to variation in color tone caused by the concave-convex structure in this portion. Further, anodic oxidation of Al proceeds by oxidation of Al in the dissolution process, but where the dust is present, dissolution also proceeds from the Al side surface of the boundary portion of the dust and Al. As a result, the electrolytic solution reaches the Ni layer, causing dissolution and generation of gas. As a result, the dust falls off, pinholes are produced, and the desired microshape is difficult to produce. Further, because the Ni layer is subjected to anodic electrlysis, swelling is caused by generation of oxygen or spots are produced by dissolution of the Ni layer and problems are associated with durability of the die.